Photographic emulsion having clayden effect susceptibility



2,7 56,148 Patented July 24, 1956 ice PHOTOGRAPHIC EMULSIUN HAVINGCLAYDEN EFFECT SUSCEPTIBILITY Edgar A. MacWilliam, Rochester, N. Y.,assignor to Eastman Kodak Company, Rochester, N. Y., a corporation ofNew Jersey No Drawing. Application June 29, 1954, Serial No. 440,282

9 Claims. (Cl. 95-7) This invention relates to photography andparticularly to a method of making a photographic emulsion which showshigh susceptibility to the Clayden effect.

Ityhas been shown that the sensitivity of some photographic materialswhen exposed by a relatively long duration exposure may be reduced ifthe materials have first been exposed to illumination of short duration,i. e. 0.001 second or less. This eifect has been calledthe Claydeneffect. The feature which characterizes the Clayden effeet is that theinitial exposure of short duration to relatively high intensity lightdesensitizes the emulsion so that subsequent exposure to weak intensitylight produces less effect than if the pro-exposure had not been given.As usually observed the density resulting from the two superposedexposures is less than that of the single exposure to the low intensitylight on making it appear that the effect is a reversal. p

The elfect known as the Clayden effect has a number of practicalapplications. Yule and Maurer have described in their application SerialNo. 291,623, now U. S. Patent No. 2,691,586, use of the Clayden eifectin preparing a pro-screened photographic material with which halftonenegatives may be prepared-from a continuous tone positive without theuse of the conventional halftone screen. The usefulness of suchpro-screened materials in the graphic arts for letterpress, lithographicand intaglio printing depends on the degree of desensitizationobtainable and hence, on the degree of sensitivity of the material tothe Clayden effect.

It is, therefore, the principal object of my invention to provide amethod for improving or increasing the susceptibility of a photographicemulsion to the Clayden effect. Other objects will appear from thefollowing description of my invention.

These objects are accomplished and the susceptibility of thephotographic emulsion to Clayden desensitization is enhanced, byprecipitating a silver chlorobromide in gelatin in such way as to formfirst a relatively chloriderich precipitate which is subsequentlyconverted to a bromide-enriched silver chlorobromide by addition ofbromide ions during at least the latter part of the precipitation cycle,preferably during at least the last /3 of the precipitation cycle of theemulsion making procedure. The emulsions ranging from 40 mole percentchloride and 60 mole percent bromide to 90 mole percent chloride and 10mole percent bromide have been found to be the most useful. I have alsofound that it is advantageous to precipitate the silver chlorobromide athigher temperature than normally used, that is, that the susceptibilityto the Clayden effect increases with iucreasing temperature ofprecipitation of the silver halide. 7 Where I refer to addition ofbromide ions or addition of alkali metal bromide to the emulsion duringat least the latter part of the precipitation cycle, I mean that thealkali metal bromide is added during or after the silver salt is addedor during the entire cycle, as long as substantially no'silver salt isadded after the addition of the alkali metal bromide is complete. It hasbeen found that I nitrate in distilled water.

the entire addition of the silver nitrate, and during at least as thetime of the addition of the alkali bromide is decreased, thesusceptibiilty of the emulsion to the Clayden effect is increased.

Instead of forming a bromide-enriched silver chlorobromide as describedabove, it is also possible to make a silver chlorobromide or silverchloride precipitate and subsequently convert the surface of the grainsthus formed to silver bromide. Useful silver chlorobromide emulsions canbe made by this technique having a chloride content ranging from 40 to60 mole percent.

According to my preferred method of making a photographic emulsion, agelatin solution of an alkali metal chloride and an alkali metal bromideis first prepared, in whichthe solution contains more than aboutone-half molar proportion of the alkali metal chloride and less than 0.1molar proportion of the alkali metal bromide. The alkali metal halidescustomarily used are the sodium, potassium or ammonium salts. Theconcentration of alkali metal halide in the gelatin may varyconsiderably; however, the range from 0.4 gram to 3.0 grams of alkalimetal halide per gram of gelatin has been found to give satisfactoryresults.

To a gelatin solution of alkali metal halide there is added during adefinite period of time of from two to ten minutes a solution of onemolar proportion of silver The solution is stirred during the latterpart of said addition of silver nitrate, an additional amount, less thanone-half molar proportion butmore than V molar proportion of alkalimetal bromide is added to the emulsion. Preferably dur'mg the additionof the alkali metal halides, these halides are kept in excess of thesilver salt; however, this is not essential. As a final step, unreactedsoluble salts are removed from the emulsion either by washing in thecustomary way or by coagulating the emulsion with a suitable gelatincoagulant, e. g., sodium sulfate, which removes the gelatin and silverhalide from the solution.

I have found that the precipitation of the silver halide should becarried out at a temperature of from 50 to C. in order to obtain theenhanced Clayden effect susceptibility. A temperature of approximately70 C. is preferred.

After addition of the second portion of alkali metal bromide, I preferto stir the emulsion'for an additional time of about ten minutes atapproximately 70 C. and remove soluble salts, then sensitize theemulsion with a sulfur sensitizer customarily used such as disclosed inSheppard U. S. Patent 1,574,944, then add additional gelatin and stirthe emulsion for an additional time, approximately 30 minutes, afterwhich it is cooled.

The emulsion may be sensitized with an optical sensitizing dye of themerocyanine or hemioxonol class as described in my application SerialNo. 440,283, filed concurrently herewith, and this also increases theClayden efiect susceptibility.

My invention will be further described by reference to the followingexamples in which the materials and conditions are merely illustrative,the invention being limited only by the scope of the appended claims.

Example 1 A solution of grams of silver nitrate in 65000. of distilledwater at 60 C. was added during 6 /3 minutes to a solution of 50 gramsof gelatin, 42 grams of sodium chloride and 1.75 grams of potassiumbromide in 1287 cc. of distilled water at 71 C. Four and one-quarterminutes after the commencement of the silver nitrate addition, theadditionof a solution of 37 grams of potassium bromide in 127 cc. ofdistilled waterat 40 C. was begun and continued for the remaining twominutes of the silver run. The resulting emulsion was stirred for '10minute s at 71 C. before the addition of 25 grams of dry gelatin soakedin water. The emulsion was stirred a further four minutes at 71 C. andwas then cooled to 29 C. Sufficient saturatedsodium sulfate solution wasadded to cause the emulsion to coagulate and settle out (about '300'cc.). The emulsion cake was rinsed three times with chilled water andthen dispersed in water at 40 C.

The emulsion was sensitized with a sulfur sensitizer such as disclosedin Sheppard U. S. Patent 1,574,944 to its maximum contrast. A furtherportion of 86grams of gelatin was then added and stirred until it wasdispersed. Before coating there was mixed with the emulsion from 0.1 to0.2 gram of l'carboxymethyl-5-[(3-ethy1'2-( 1)-benzotxazolylidene)--ethylidene]-3-phenyl-2-thiohydantoin dissolved in1% alcoholic'tiethylamine solution.

In order to testthe Clayden eflect susceptibility the emulsion aftercoating was exposed through a step tablet with 0.15 density incrementsto illumination from a Kodatron flash lamp at 60 inches. The emulsionwas then exposed through a similar step tablet set at right angles tothe first to lower intensity illumination for 0.1 second. The emulsionwas then developed for 2 /2 minutes at 68 in a developer of thefollowing compo- Example 2 Three chlorobromide emulsions were made asdescribed in Example 1 except that the temperatures of precipitation ofthe silver halide were 55 C., 71 C. and 78 C. respectively, and theemulsions were coated with 0.1 g. of sensitive dye. Upon testing thecoatings as described in Example 1, the extent of Clayden effect desensitization increased as the temperature of precipitation wasincreased, as shown in the following table:

Deserts. ALog E Measured. at a Density of 1.0

Temperatureol' Procipltatlon Example 3 Three chlorobromide emulsionswere made as described in Example 1 exceptthat the second potassiumbromide solution was added during the last six minutes, the last fourminutes and the last two minutes, respectively, of the silver nitraterun, so that the silver and bromide runs ended at approximately the sametime. Upon testing the coatings as described in Example 1,susceptibility to Clayden elfect was found to be increased as "more ofthe additional potassium bromide was added during the latter part of therun, as shown in the following table:

Desens. ALog E Time of Bromide Addition Measured at a Density of 1.0

(minutes 0. 40 4minutes. 1. 05 2minutes 1. 70

What I claim is: 1. The method of making a photographic emulsion havinga high susceptibility to, Clayden desensitization,

which comprises adding one molar proportion of a soluble silver salt toa gelatin solution of more than about one-half molar proportion of asoluble chloride and less than about 0.1 mole of a soluble bromide, withstirring, during less than ten but more than two minutes, and during aperiod of time beginning after the commencement of said addition ofsilver salt, adding an additional amount, less than one-half molarproportion but more than 0.1 molar proportion, of a soluble bromide tosaid metal bromide to said solution, and removing unreacted solublesalts from said emulsion.

3. The method of claim 1, in which the addition of the alkali metalhalides is carried out at approximately 50 C. to C.

4. The method of claim 1, in which the addition of the alkali metalhalides is carried out at approximately 70 C.

5. The method of claim 1, in which addition of silver nitrate is madeduring six minutes, and addition of alkali metal bromide is made duringthe last five minutes of the silver nitrate addition.

6. The method of claim 1, in which the emulsion is stirred for anadditional ten minutes at approximately, 70 C. after addition of thesecond alkali metal bromide, additional gelatin is then added, and theemulsion is stirred for an additional four minutes and then cooled toapproximately 30 C.

7. The method of claim 1, in which the emulsion is cooled toapproximately 30 C. after addition of the sec ond alkali metal bromide,and is then coagulated with sodium sulfate, rinsed with water, anddispersed in water.

8. The method of claim 1, in which the final emulsion is sensitized witha sulfur sensitizer.

9. The method of claim 1, in which the final emulsion is sensitized with1-carboxymethy1-5-[(3-ethyI-2-(1)-benzoxazolylidene)-ethylidene]-3-phenyl-2-thiohydantoin.

References Cited in the tile of this patent UNITED STATES PATENTS1,574,944 Sheppard Mar. 2, 1926 FOREIGN PATENTS 545,944 France Aug. 5,1922 580,173 Great Britain Aug. 29, 1946 690,997 Great Britain May 6,1953 GTHER REFERENCES Wall: Photographic Emulsions (1929). Pub. by

American Photographic Pub. Co., Boston (pages 98 and 99).

1. THE METHOD OF MAKING A PHOTOGRAPHIC EMULSION HAVING A HIGHSUSCEPTIBILITY TO CLAYDEN DESENSITIZATION, WHICH COMPRISES ADDING ONEMOLAR PROPORTION OF A SOLUBLE SILVER SALT TO A GELATIN SOLUTION OF MORETHAN ABOUT ONE HALF MOLAR PROPORTION OF A SOLUBLE CHLORIDE AND LESS THANABOUT 0.1 MOLE OF A SOLUBLE BROMIDE, WITH STIRRING, DURING LESS THAN TENBUT MORE THAN TWO MINUTES, AND DURING A PERIOD OF TIME BEGINNING AFTERTHE COMMENCEMENT OF SAID ADDITION OF SILVER SALT, ADDING AN ADDITIONALAMOUNT, LESS THAN ONE-HALF MOLAR PROPORTION BUT MORE THAN 0.1 MOLARPROPORTION, OF A SOLUBLE BROMIDE TO SAID SOLUTION.